1. Field of the Invention
The present invention relates to a liquid-discharging apparatus including a head equipped with a plurality of juxtaposed liquid-discharging units having respective nozzles, forming dots by landing droplets discharged from the nozzles onto a droplet-landing object, and providing half tones by arranging a dot array, and also relates to a density adjusting method and a density adjusting system for adjusting the density of the dots. More particularly, the present invention is relates to a technique for adjusting density unevenness when the unevenness occurs due to a variation in discharging characteristics of the liquid-discharging units.
2. Description of the Related Art
An inkjet printer is known as one of conventional liquid-discharging apparatuses. The inkjet printer is equipped with a head including a large number of juxtaposed liquid-discharging units having respective nozzles, forms dots on a sheet of printing paper by discharging ink droplets from the nozzles, and forms an image by arranging arrays of the dots.
Also, a serial-type inkjet printer performs printing in the main scanning direction (in a direction perpendicular to a feeding direction of a sheet of printing paper by using a known method (see, for example, Japanese Examined Patent Application Publication No. 56-6033) for providing half tones by superimposing dots by reciprocating the head more than once, that is, by applying so-called overprinting. To be specific, according to the method, at every movement of the head in the main scanning directions the first recording is performed with a dot pitch greater than the diameter of a dot, and the second recording is performed by arranging a dot so as to cover the space between adjacent dots generated in the first recording.
With the above-mentioned overprinting for providing half tones, discharging characteristics of the liquid-discharging units are made more uniform, thereby making density unevenness indistinctive. Meanwhile, when the head has a plurality of liquid-discharging units juxtaposed side by side therein, a variation in discharging characteristics of the liquid-discharging units, for example, a variation in discharge amounts of ink droplets occur. Unfortunately, the head of the inkjet printer, for example, including thermal liquid-discharging units, can discharge only a constant amount of ink droplet from each nozzle during one discharging operation, except for a special head including a special discharging mechanism formed by utilizing the piezo technology. In other words, a discharge amount of an ink droplet during one discharging operation cannot be controlled.
As a countermeasure for solving the above disadvantage, overprinting is applied so a to make density unevenness substantially indistinctive even when a part of the liquid-discharging units have poor discharging characteristics, for example, discharging an insufficient amount or no amount of ink droplet due to clogging of the corresponding nozzles or the like.
Unfortunately, according to the above-mentioned overprinting method, problems such as density unevenness caused by a variation in discharging characteristics of the liquid-discharging units can not be completely solved.
Firstly, a problem arises from a certain limitation of an ink-absorbing amount of a sheet of printing paper. That is, when a dot is superimposed beyond the limitation of an ink-absorbing amount of a sheet of printing paper, the dot is unlikely dried, and also, to make matters worse, ink of the dot spreads over the adjacent dots and generates color mixture with that of the adjacent dots, thereby leading to a failure in achieving an expected density gradation characteristic.
Secondly, when high image quality, for example equivalent to that of a photographic image is required, existence of even a small part of the liquid-discharging units of the head which do not normally discharge ink droplets makes a streak or the like distinctive. For example, when a color other than black is printed in a pupil area in the case of printing an image such as a facial portrait, or when a color other than red is printed in an apple or flower area in the case of expressing such an object, the foregoing color becomes distinctive even when its printed area is tiny.
In order to solve such density unevenness, a thermal sublimination printer or the like normally having a line head structure has an example countermeasure incorporated therein as described below.
FIG. 21 illustrates a general method for correcting density unevenness by image processing. A density measuring-pattern (test pattern) providing a uniform and constant density is first printed so as to measure a state of density unevenness with respect to each color across the full sheet of paper. Then, the printed result with respect to each color is scanned by an image-scanning apparatus. Since the scanned data includes density information and unevenness information, the average density and coefficients of unevenness over the all pixels are computed. In addition, a data table obtained by multiplying all positions corresponding to the pixels of an input image by the reciprocals of coefficients of unevenness corresponding to the positions (that is, obtained by computation with an inverse function) is produced and stored.
When an image is inputted, multiplication process is performed on the basis of the data table before image processing so as to produce a corrected image file, and a printing operation is performed on the basis of information of the corrected image file, whereby density unevenness peculiar to the head is canceled.
Meanwhile, this method is presently used for printers other than an inkjet printer, and it will be appreciated that it is also applicable to an inkjet printer.
Unfortunately, the foregoing known method for correcting density unevenness is needed to process an input image, and, especially when an input image including a large amount of data is required to be processed, a longer period of time for processing the input image is needed before printing, thereby resulting in a reduced printing speed.
Improvement in the printing speed incurs an increase in hardware, memory, and the like, and hence causes a larger size of the printer.